Solid state lighting devices such as, for example, light emitting diodes (LED's) are used for a number of applications. One type of such solid state lighting device is disclosed in International Patent Application No. PCT/US03/14625, filed May 28, 2003, entitled High Efficiency Solid-State Light Source And Methods Of Use And Manufacture, the details of which are hereby incorporated by reference.
There are numerous applications where a long string of devices, such as, for example, LED's, need to be connected electrically. Such strings present unique problems for the electrical engineer. On the one hand, there is a desire to string the components in series so that the current from one component flows directly through the next component. This is a desired configuration because it minimizes the amount of electrical current required while increasing the total voltage required across all the components. Since high currents are more difficult to deal with because high currents require large gauge wires, for example, it is desired to have lower currents and higher voltages.
However, stringing the components together in series presents a problem because if one of the components in the string fails, it will result in the failure of the entire string. For example, in a string of holiday lights wired in series, if one light fails the entire string also fails. To overcome this problem, holiday string lights are typically wired in parallel so that when one light fails the rest of the lights in the string continue to operate. However, such wiring requires higher current and lower voltage.
Wiring lights in series is preferred because the total current is lower and the operating voltage is higher. This presents a problem because if one light fails all lights in the series fail. Wiring lights in parallel overcomes this problem because when one light fails all other lights still operate. However, one undesirable aspect of wiring in parallel is that the total current is higher and the operating voltage is lower.
One prior art approach to this problem is described in U.S. Pat. No. 6,153,980 (Marshall et al). This patent describes a circuit that has individual sensors for each light source and can determine if any given light source has failed. In the event of failure, the circuit shunts current around the failed component so that the rest of the components that are wired in series continue to receive electrical current. While such a circuit solves the problem of allowing serial connection (and, thus, higher voltage and lower current) the circuit itself is more complicated, expensive, and prone to possible failure, which defeats it's intended purpose.
What is needed is a light source that never fails or that at least has such a high reliability and mean time between failures that failure is something that effectively can never happen. Thus, the preferred solution changes from parallel wiring to series wiring forming a cascading series parallel circuit substantially reducing failures and mean time between failures. The parallel/series circuitry enables the selection of current and potentials that can accommodate the specific performance of solid state light sources in addition to complying with industry standards for different markets. These markets can be, but are not limited to industrial (high power), consumer (low power) and specialty markets as in the case of aerospace and medical markets.